PROJECT SUMMARY/ABSTRACT High intensity focused ultrasound (HIFU) modalities, such as histotripsy, have been used successfully to emulsify soft organs, like the heart and liver, but highly collagenous tissues, like tendons, have proven resistant to mechanical emulsification with histotripsy. Recently, histotripsy was shown to initiate an immune response and release biomarkers. If histotripsy can successfully emulsify collagenous tissues, it has the potential to initiate a healing response by inducing inflammation and subsequently release healing factors. With the high clinical prevalence of tendon injuries, establishing whether histotripsy can create microdamage in collagenous tendons to promote healing has the potential for long-term impact. Rotator cuff tear is a highly prevalent cause of shoulder pain, which is why patients seek medical treatment. Surgical repair is a common treatment for rotator cuff tear, but fails up to 90% of the time, likely from the high mechanical loading the rotator cuff experiences in its dual roles with joint mobility and stability. Others have used dry needling (DN), which involves repeated puncturing of the tendon with a fine-gauge needle to induce microdamage and release healing factors, but adoption of this approach is not wide-spread. Development of techniques that can noninvasively promote healing while maintaining mechanical integrity of the muscle-tendon units are desperately needed. Demonstration of histotripsy therapy for collagenous tissue emulsification would provide a clinically transferrable tool to immediately supplement current treatment options of a contemporary clinical problem. Here, we propose to establish non-invasive histotripsy protocols that produce better indicators of tendon healing compared to DN, while simultaneously maintaining tendon's mechanical properties. These goals will be accomplished by: 1) testing novel histotripsy protocols to induce microdamage in collagen gels and ex vivo rat tendons; 2) evaluating the mechanical properties of tendon following histotripsy and DN, and develop a finite element model to evaluate parameters and perform predictive analyses; and 3) determining whether histotripsy enhances collagenous tissue healing in vivo in a pilot survival study in rats. This innovative work seeks to demonstrate that histotripsy can successfully emulsify collagenous tissues, and enhance the release of healing factors. Once feasibility has been demonstrated, the methods and models developed in this project will be scaled to examine histotripsy in larger animal models and humans. Long term, outcomes will lead to histotripsy emulsification of other collagenous tissues as well as clinical translation of tendon healing protocols, which has the potential to revolutionize treatment for rotator cuff tear and other collagenous tissue injuries.